3d reverse printing method and device

ABSTRACT

The present invention relates to a method and a device for producing three-dimensional models, wherein binding/bonding material is applied in layers to a building platform and media are selectively applied which delay or completely prevent the binding of the applied material.

The invention relates to a method and a device for producingthree-dimensional models, wherein binding/bonding material is applied inlayers to a building platform and media are selectively applied whichdelay or completely prevent the binding of the applied material.

A method for producing three-dimensional objects from computer data isdescribed, for example, in EP 0 431 924 B1. In this method, aparticulate material is deposited in a thin layer onto a platform, and abinder material is selectively printed on the particulate material,using a print head. The particle area onto which the binder is printedsticks together and solidifies under the influence of the binder and, ifnecessary, an additional hardener. The platform is then lowered by adistance of one layer thickness into a build cylinder and provided witha new layer of particulate material, which is also printed as describedabove. These steps are repeated until a certain, desired height of theobject is achieved. A three-dimensional object is thereby produced fromthe printed and solidified areas.

3D printing methods are furthermore known, in which areas areselectively printed with media for the purpose of preventing a sinteringof these areas, and the unprinted areas are then permanently bonded toeach other in a sintering step, whereby a three-dimensional model isultimately created. One example of this is WO 01/38061 A1, which neitherdiscloses nor suggests the invention.

The known 3D printing methods may be used to process differentparticulate materials, such as natural biological raw materials,polymers, plastics, metals, ceramics and sands.

Binders that are used outside of 3D printing applications are highlydeveloped. Epoxy resin may serve as an example. When mixed with sand,for example, it results in a stone-like material with excellent strengthproperties.

Up to now, however, making material systems which are intrinsicallyadvantageous for 3D printing and have very positive material propertiesaccessible to this method has been unsuccessful either entirely or onlyto an unsatisfactory degree. These positive material properties are thuslost to 3D printing. The print head process component, which is able toprocess only a very limited range of materials and is susceptible toerrors when using known and intrinsically advantageous materials, isproblematic in this context.

Specifically, 3D printing methods are subject to the disadvantagesdescribed below.

Powder-based 3D printing methods are described, for example, in patentEP 0 431 924 B1. In this case, a base material in the form of powder isapplied in layers. Cavities naturally result between the particles.These cavities weaken the strength of the printed component.

To increase the strength, the cavities must be filled with material thatis introduced through the print head in liquid form. The printedmaterial must harden without drying. Since the material is dosed via theprint head, relatively low viscosities in the liquid state arenecessary, especially in the ink jet methods used here.

For example, if we examine the material system of concrete, we will seethat concrete mixed at the point of use solidifies on its own withoutsupplying any air. However, the cement paste per se cannot be processedwith the aid of an ink jet print head.

Reactive resin systems may be a second example. These materialspolymerize, e.g., after the mixing of two reactive components, and thusharden. The chain length of the prepolymers is usually very high in thiscase in order to minimize shrinkage during the reaction. The healthhazard decreases as the chain length of the prepolymers increases,although the viscosity also increases. Such material systems are thususually difficult or impossible to print.

Another feature of such products is that they require intimate mixingprior to casting. Only by doing so is the right reaction between thefunctional groups in the viscous medium ensured. A mixture of this typewould have to take place prior to the dosing operation and would thusendanger the print head, since the mixture may harden in the print headbefore it is dosed.

One objective of the invention is to make the use of standard materialshaving positive material properties accessible to 3D printing or toavoid or at least reduce the disadvantages of the prior art.

The approach according to the invention is achieved in one aspect by amethod for producing three-dimensional models using a layeringtechnique, wherein particulate build material is applied to a buildspace in a defined layer thickness in the form of powder or dispersion,a liquid is then selectively applied via a print head to the buildmaterial in the areas in which the build material is not to besolidified by a solidification reaction, the build space is moved,preferably lowered, raised or horizontally moved, by the layerthickness, and these steps are repeated until the desired model isproduced, wherein the solidification reaction of the build material hasalready started, starts or may be started by introducing energy upon theapplication of the build material, and a reaction-inhibiting orreaction-stopping liquid is selectively applied by the print head, andno binder material is preferably applied and/or no sintering step iscarried out.

In one aspect, the invention essentially consists in making the mixingand consolidation methods known from material processing usable for 3Dprinting when using standard materials by applying a method in which thenegative of the necessary mold is treated in layers with a retardingagent.

The advantage of the method according to the invention is that materialsystems having advantageous material properties may now be madeaccessible to 3D printing. It is thus possible to produce 3D modelshaving very good molding properties cost-effectively and in atime-saving manner.

The advantage of the invention also consists in the fact that a buildingmaterial which solidifies over time may be applied in layers, and a 3Dmodel having good material properties may thus be produced withoutrequiring any additional treatment or solidification steps, such assintering.

The present invention and, in particular, the method according to theinvention, relate to a 3D printing method with the proviso that thelatter does not involve any sintering step and preferably uses materialsystems or materials other than those in WO 01/38061 A1.

The difference between the invention described in the present case and,for example, WO 01/38061 A1 is, in particular, that the method describedtherein relates to a sintering method. A method having a sintering stepis preferably excluded from the scope of protection of the presentinvention or represents a disclaimer.

A number of terms in the invention are explained in greater detailbelow.

Within the meaning of the invention, “3D printing method” or “layeringtechnique” relates to all methods known from the prior art whichfacilitate the construction of models in three-dimensional forms and arecompatible with suitable method components and devices. In particular,these are powder-based methods, which may be carried out using suitablematerial systems.

“Molded body,” “model” or “component” within the meaning of theinvention are all three-dimensional objects that are produced with theaid of the method according to the invention and/or the device accordingto the invention and which have a nondeformability.

Any known 3D printing device that contains the necessary components maybe used as the “device” for carrying out the method according to theinvention. Common components include a coater, a build space, a meansfor moving the build space or other components, a dosing device and aheating means and other components which are known to those skilled inthe art and therefore do not need to be listed in greater detail here.

All materials in powder form, in particular sands, ceramic powders,metal powders, plastics, wood particles, fibrous materials, cellulosesand/or lactose powders may be used as “particulate materials.” Theparticulate material is preferably a dry, free-flowing or a cohesive,firm powder. Dispersions of the particulate materials employed may alsobe used.

“Build space” is the geometric place in which the particulate materialfeedstock grows during the build process by repeated coating withparticulate material. The build space is generally delimited by a floor,the building platform, by walls and an open cover surface, the buildplane.

“Building platform” within the meaning of the invention is the plane onwhich the model is built. It may be moved after each layer applicationduring the construction of the model. The building platform may bemounted in the device essentially horizontally or at an angle.

A “moving” of the building platform means that the building platform islowered, raised or moved horizontally by one layer thickness in order toapply the next layer.

“IR heating” in this patent means an irradiation of the build spaceusing an infrared emitter. The emitter may be static, or it may be movedover the build space with the aid of a positioning unit.

“UV hardening” refers to the initiation of a material system by means ofradiation. The wavelength of the radiation does not necessarily have tobe in the spectrum of the UV radiation. Wavelengths of the UV-Vis andVis classes may also be used. A radiation of this type may be used asthe activation step.

“Build material” within the meaning of the invention is any material ormaterial mixture that may be applied with the aid of a device within themeaning of the invention. These may be powders or particulate materials.They are preferably two-component mixtures, in which a solidificationreaction is started only by mixing them together. The build material mayalso be present in the form of a dispersion. Materials or buildmaterials or a material system within the meaning of the invention mayconsist, in particular, of concrete or mortar, a mixture of a filler anda reactive resin system, a mixture of a foundry molding material and areactive resin system which is common in foundries, a mixture of afiller and water glass or a mixture of particles and meltable material.Build materials within the meaning of the invention are described inmore detail below.

“Layer thickness” within the meaning of the invention is setselectively, depending on the material and special methodimplementation. It may remain the same throughout the method or bevaried during the course of the method. The layer thickness ispreferably in the range of 50 to 500 μm, more preferably 100 to 400 μmand even more preferably 150 to 300 μm.

The “solidification reaction of the build material” within the meaningof the invention has already begun when the build material is applied.It may be accelerated by suitable measures, such as regulating thetemperature of the build space, or initially started by an activationmeasure. An application of energy in the form of heat or thermalradiation may be used for this purpose.

A “reaction-inhibiting” or “reaction-stopping material” or “liquid”within the meaning of the invention—also referred to as a “retardingagent”—is any material which is able to slow down, inhibit or entirelystop a solidification of the building material by being applied. A“retarding agent” within the meaning of the invention is a substancethat influences and slows down the hardening process. These includebases, acids, alcohols, hydrophobic solutions, oils, hydroquinone, orsubstances having monofunctional groups.

“Binding agents” within the meaning of the invention may preferably be acement or gypsum, an acrylate or styrene, a polyurethane, an epoxyresin, a polyester and/or a polyamide. Additional details are discussedbelow.

An “activation step” within the meaning of the invention is a methodmeasure which causes the solidification reaction to progress fasterand/or more completely. In one preferred implementation of the method,an activation step may also mean a crossing of a threshold which iscritical for the particular reaction, which results in the controlledstarting of the solidification reaction.

Reactions within the meaning of the invention may preferably be—withoutbeing limited thereto—a hydration, a polymerization or a phasetransition reaction.

The mechanism according to the invention is based in part on the“retardation” or blocking of the hardening process. A duration of theretardation which makes it possible to safely remove deposits from thecomponent is meant hereby. In the case of activatable materials, theduration is nearly unlimited.

“Deposits” are partially solidified areas which occur outside thegeometric boundary of the component. They are generally undesirable andshould be removable by means of brushing, blowing off or powderblasting.

“Outer area” describes the surface outside the geometric boundary of thecomponent. According to the invention, a retarding agent is appliedhere. This may take place, according to the invention, over the entiresurface or only on the component boundaries or outside the component ineach layer, or partially.

“Melting temperature” within the meaning of the invention relates to atemperature which must be present in the build space and ultimately inthe build material in order for a solidification process to result. Itmay vary and depends on the particular materials used.

In the method according to the invention, the particulate material(build material) is preferably selected from the group consisting offreshly mixed concrete or mortar, a mixture of a filler and a reactiveresin system, a mixture of a foundry molding material and a reactiveresin system common in foundries, a mixture of a filler and water glassor a mixture of particles and meltable material, the mixture beingheated to a temperature above the melting temperature of the meltablematerial prior to application.

In the method, the build material is normally mixed intimately in ameans provided for this purpose shortly before being applied, and ahardening reaction then independently sets in. Additional activationsteps or solidification measures, such as sintering, are no longernecessary to obtain the hardened component.

However, it may be advantageous to additionally carry out an activationstep to accelerate the method and the build process for producing the 3Dmolded parts. This activation step preferably takes place after theapplication from the print head.

The additional activation step preferably takes place by means ofirradiation, more preferably with the aid of IR radiation.

It may be advantageous for different reasons, for example to obtainadvantageous material properties in the 3D components, to delay thesolidification reaction in the method according to the invention byhours, particularly preferably by multiple days, or to essentially stopit altogether.

In another preferred embodiment, the method and the materials forbuilding the 3D molded part may be selected in such a way that thesolidification reaction is started by radiation.

The solidification reaction may be based on different mechanisms,depending on the selected build materials. The solidification reactionmay be a hydration, a polymerization or a phase transition reaction.

Any suitable binding agent may be used which is compatible with theother material components and device means. A cement or gypsum, anacrylate or styrene, a polyurethane, an epoxy resin, a polyester and/ora polyamide is/are preferably used as the binding agent.

In another aspect, the invention relates to the use of a materialsystem, as described herein, in a 3D printing method described herein.

The invention furthermore relates to a device for carrying out a 3Dprinting method according to the invention.

In another aspect, the invention relates to a device which includesmeans known to those skilled in the art for carrying out a 3D printingmethod. This device has particular device means, modified in a specialmanner or additionally, to be able to carry out the method according tothe invention.

Those skilled in the art will understand that precautions must be takento carry out the method according to the invention, in order to ensure afault-free operation, for example, of the print head.

The print head may be specially designed in different aspects for thispurpose, such as being coated with non-stick media to prevent the printhead from sticking. For example, a Teflon coating of the print head maybe provided in this case.

Another advantageous embodiment of the printing device may consist inthe fact that the device includes a cleaning station for the print headand/or the coating means (coater, recoater). The print head and/or thecoater is/are preferably moved to this cleaning station after eachcoating, more preferably after every tenth coating, or as needed, and isfreed of contaminants of the applied materials (buildmaterial/particulate material, material mixture, reaction-inhibiting orreaction-stopping liquid). At the cleaning station, contaminants areremoved from the coater along its entire length.

In the method according to the invention, build material which isalready capable of binding or bonding or hardening, or whose materialcomponents pass through a reaction that permits a binding of theparticle components, is already situated in the coater.

One approach for keeping the coater operational and protecting itagainst contaminants or from being clogged with binding materialtherefore consists in the fact that the device has a so-called dumpstation, to which the coater is moved while other method steps are beingcarried out, or there is, for example, a coating pause during the courseof the method. This dump station is designed in such a way that thecoater continues the coating process here and thereby ensures that thebuild material is continuously dispensed, and a clogging by the buildmaterial is prevented.

Once the coating operation is to be continued in the method, the coateris returned to the building platform, and the build process of the 3Dmodel may be continued.

The method according to the invention furthermore uses build material,in which the binding reaction/solidification reaction has alreadystarted when the coater is filled. To minimize the so-called deadmaterial, which is ultimately not used in the build process, thematerial components are mixed with each other using a mixing means ashort time prior to filling the coater. This has multiple advantages.Due to the relatively small amounts of this mixture, an intimate mixingas well as a good reaction and thus solidification in the build materialis ensured, which has a positive effect on the material properties ofthe 3D component produced. In addition, the amount of dead material inthe material to be discarded is kept as small as possible, and the useof material is thus also minimized.

As a result, in one preferred embodiment, the device comprises acleaning station for the coater and, if necessary, for the print head, amixing device for the build material and preferably a dump station fordischarging dead material, in addition to the usual structural means ofa 3D printing device.

In another preferred aspect, the invention thus relates to a device,suitable for 3D printing, comprising a movable building platform, acoater for applying build material which is a self-hardening materialmixture, at least one print head unit, at least one mixing unit, atleast one cleaning unit for cleaning the coater, if necessary, at leastone cleaning unit for cleaning the print head unit and, if necessary, areceiving unit for receiving build material.

The receiving unit is preferably used to receive dead material which isnot used for the build process, while the coater must be moved into awaiting position due to other work steps to be carried out. Bydischarging build material from the coater into this receiving unit, itis advantageously achieved that the coater is not contaminated, remainsfully operational, and the coating may continue in a precise mannerafter the waiting position and waiting time. This also partially avoidsthe need for cleaning, or at least reduces the cleaning cycles. This hasa positive effect on the process speed and increases the productivity ofthe build process, while maintaining the same standard of quality.

BRIEF DESCRIPTION OF THE FIGURES

The figures describe 3D printers from the prior art as well as preferredspecific embodiments of the invention.

FIG. 1: shows a schematic representation of the components of apowder-based 3D printer in an isometric view.

FIG. 2: shows a sequence of a conventional 3D printing process with theuse of a layered radiation hardening technique.

FIG. 3: shows a diagram of the unpacking of components from areaction-inhibited material which may be used according to theinvention.

FIG. 4: shows a diagram of an inhibited phase transition reaction whichmay be used according to the invention.

REACTION SCHEMATA

Schema 1: Chemical reaction of a reaction-inhibited cold resin methodwhich may be used according to the invention.

Schema 2: Chemical reaction of a reaction-inhibited cold-box methodwhich may be used according to the invention.

Preferred Embodiments of the Invention

In a first step, a liquid or powdered material (build material) isapplied to a building platform and smoothed with the aid of a coater.This material solidifies on its own over time or preferably by means ofan activation process. The material may solidify, for example, by meansof drying. A chemical reaction that progresses slowly is alsoconceivable. Physical processes, such as a phase transition, are alsoconceivable.

A liquid substance, or a substance located in a liquid, which slows downor blocks the particular solidification mechanism, is then applied viaan ink jet print head (print head) before or after an optionalsolidification step.

A solidification step may be, e.g., a drying process following the printprocess. However, a UV activation of a hardenable polymer is alsoconceivable.

The building platform is subsequently lowered by one layer thickness.

The aforementioned steps are repeated until the desired body has beencreated.

In the end, the material surrounding the component and the deposits areremoved. This may take place by brushing, blasting or rinsing, dependingon the type of the mechanism.

The advantages of this method are that the slowing down or blocking ofthe solidification mechanism may usually be achieved using a very smallamount of an inhibiting medium. This medium is usually easier to dosethan the solidifying materials.

In addition, intimate mixtures may be achieved in this manner. Thematerial may be intensively prepared using the customary mixing tools.Powerful mixing energies may be used.

The third aspect relates to the introduction of reinforcements into thecomponent. Due to the premixing, much more complex materials may beprocessed as a layer than when using prior-art methods.

The consolidation of the base material is likewise influenced. Methodsfor the liquid dispensing of powder materials may be used, whichadvantageously influence an efficient particle packing.

In summary, the inventors have developed a means for advantageouslymaking the use of materials accessible to 3D printing methods which werepreviously unusable for 3D printing. For example, a conventionalconcrete may be processed.

The result is a component which is in no way inferior to a cast quality.For example, a plastic component may likewise be produced from highlyviscous reactive resins. Strengths that are excellent for additivelyproduced components are associated therewith.

Other Preferred Embodiments of the Invention

The system according to the invention draws heavily on powder-based 3Dprinting. The mechanical engineering is augmented to meet therequirements according to the invention.

The device according to the invention includes a coater (2). This coateris used to apply and smooth premixed particulate material or a liquidcontaining particulate material onto a building platform (3) (FIG.2(a)). The applied particulate material may consist of a wide range ofmaterials. For example, sands, ceramic powders, metal powders, plastic,wood particles, fibrous materials, celluloses, lactose powders, etc. maybe used. The flow characteristics of these materials may varyenormously. Different coater techniques permit layering from dry,free-flowing powders and cohesive, firm powders to liquid-baseddispersions. The height of powder layers (4) is determined by buildingplatform (3). It is lowered after one layer has been applied. During thenext coating operation, the resulting volume is filled and the excesssmoothed. The result is a nearly perfectly parallel and smooth layer ofa defined height.

According to the invention, the solidification process of the appliedbuild material begins before the application, since all componentsneeded for the reaction have been recently mixed intimately in a mixingmeans. The build material is thus produced in a preparation means priorto dispensing and quickly fed to the coater. The transport times andflow rates are monitored. If an error occurs, the material is notsupplied to the machine, and the machine is rinsed with neutralmaterial. The neutral material may comprise, for example, a passivecomponent of the build material.

The material feed is structurally carried out without any dead space.I.e., the material flow always carries along all material quantitieslocated in the feed laminarly.

After a coating process, the layer is printed with a liquid—theso-called retarding agent—with the aid of an ink jet print head (1)(FIG. 2(b)). The print image corresponds to the inverted section of thecomponent in the present build height of the device. The liquid slowlyand diffusively penetrates the particulate material.

Following the method according to the invention, the retarding agentsolidifies the layer during or shortly after printing (FIG. 2(c)). Tospeed up this process, or to initiate the hardening, an IR radiator (5),for example, may be additionally passed over the build space in onepreferred embodiment. This IR radiator may be coupled with the axis ofthe coating system. The solvent evaporates during heating. In the caseof liquids that present a fire hazard, the evaporating material isextracted immediately (7).

This process may be used to influence, and preferably to speed up, thetime sequences of the solidification reaction. The period of time untilthe parts are unpacked may be shortened thereby.

In the method according to the invention, a job block (300) is generallyproduced by the build process, from which the embedded components (301)must be removed following the build process. This procedure may takeplace, e.g., by rinsing with a liquid (302, 303). Likewise oradditionally, the component may be exposed by scrubbing or brushingmanually (305).

Brief descriptions of preferred material systems are provided below. Ineach case, the overall process is briefly illustrated.

Cement-bound material An aggregate, such as sand, is mixed with a cementpaste, water and additives. This paste-like substance is applied to abuilding platform with the aid of a special coater.

An acidic sugar solution is dispensed as the reaction-retardingsubstance onto the areas that are not to be solidified. This combinationinterferes with the hydration of the cement and thus the solidification.

After a certain binding time, the resulting job block may be detachedwith the aid of water. The “retarded” material is then brushed off underthe action of water. The rinsing solution is environmentally safe, sinceit is non-toxic.

The component produced in this manner is comparable to a cementcomponent produced by casting in terms of its condition and strengthcharacteristics.

Polymer Synthesis Reaction (Cold Resin)

A reaction-retarded cold resin binding agent for metal castingapplications is mixed with a hardener and sand as a batch and fed intothe coater. The latter applies the build material as a layer. Comparedto the prior art, a special coater is used, which is able to processvery cohesive sand mixtures.

An ink-jet print head prints a basic substance in the area in which thesand is to be removed later on. Only a small quantity of a highlybasic-acting substance must be applied.

The build material completely solidifies in this process without asubstance being printed thereon. Special measures must be taken toprotect the mixer, coater and build container.

The result of this process is a porous mold or a core having a coldresin binding, which may be used in metal casting in the known manner.

Polymer Synthesis Reaction (Polyurethane)

An at least difunctional prepolymer isocyanate is mixed with at leastdifunctional phenol-containing polyol and other additives and withfoundry sand. The mixture is set in such a way that the polymerizationreaction takes multiple tens of minutes or sets in only after at leastthis dwell time.

In the area where the sand is to be removed later on, an ink-jet printhead prints a substance which, on the one hand, is monofunctional and,on the other hand, reacts with a reactive component much faster thanwith the multifunctional reactant. The reactive groups needed for apolymerization are reduced thereby. In this system, for example, aprinting of compounds with the formula R—OH or R1-NH—R2 is efficacious,short-chain alcohols, such as ethanol or 2-propanol being preferred withregard to the resulting reaction products. Isocyanate groups areeffectively and irreversibly deactivated with these substances.

For unpacking, the loose sand is brushed and blasted off.

The result is a casting mold for metal casting applications of the coldbox type known in this field.

Polymer Synthesis Reaction (Polyacrylate)

A base material, such as expanded glass or a polymer, is mixed withradically polymerizable acrylates. A photoinitiator is also added to thesystem.

In the area where the base material is to be removed later on, anink-jet print head prints a substance which interferes with the furtherconcatenation or deactivates the photoinitiator. These may be knowninhibitors such as TEMPO or hydroquinone.

The printing process is followed by an exposure to light using a UVlamp. Due to the deactivated photoinitiator, either no radicals for thepolymerization, or only as many radicals as can be immediately absorbedby the added inhibitors, are formed in the printed areas. As a result,no solidity may build up in the printed areas.

The result of this process is a filled plastic component. The latter maybe used as a function component in industrial applications or as avisual aid.

Phase Change

A base material (600), e.g. polystyrene powder, is mixed with a wax(601) and heated to a temperature above its melting point (FIG. 4a ). Inthe hot state, it is applied to the building platform with the aid ofthe coater. The temperature in the build space is maintained above themelting point of the wax.

This layer is printed with oil drops (602), e.g., paraffin oil (FIG. 4b). This oil is mixable with the hot wax. This mixture (603) is locatedbetween the particles in the outer area.

During the slow cooling process, a solidity forms in the unprinted area,due to the cooled particle bridges (604). In the printed area, the oilinterferes with the formation of solidity. The particles in this area(outside component contour 605) may subsequently be easily brushed off.

The component produced in this manner may be used as a so-called waxmodel for investment casting applications.

To simplify the separation of the desired component and the surroundingmaterial, printing the retarding agent only linearly along the outercontour of the particular layer cross section and printing the areasoutside the component contour with a grid are expedient for reasons ofmaterial economy. If all-over separating planes are now inserted in acertain order, a cube structure arises around the component, which maybe easily removed, even from complex geometric sections. The size of thecube geometry may be either set as standard, or it may automaticallyadapt to the component contour with the aid of corresponding algorithms.

A number of reaction schemata are furthermore illustrated for thematerial systems according to the invention:

Reaction of furfuryl alcohol with condensation under acidic conditions;increase in pH value prevents the reaction from taking place.

Polymerization is prevented by a competition reaction of the isocyanatewith 2-propanol.

List of Reference Numerals

-   100 Ink-jet print head-   101 Powder coater-   102 Building platform-   103 Component-   104 Build space boundary-   107 Powder layers-   200 Solidifying unit-   300 Job block-   301 Embedded component-   302 Rinsing nozzle-   303 Dripping material in the outer area-   304 Exposed component-   305 Brush-   600 Base particles-   601 Liquid wax-   602 Oil drops-   603 Bridge of wax/oil-   604 Solidified wax-   605 Component contour

What is claimed is:
 1. A method for producing three-dimensional modelsusing a layering technique, comprising the steps of: applying aparticulate build material is applied to a build space in a definedlayer thickness in the form of a powder or a dispersion, selectivelyapplying a liquid via a print head to the particulate build material inone or more areas in which the particulate build material is not to besolidified by a solidification reaction, and moving the build space,wherein these steps are repeated until the desired model is produced,wherein the solidification reaction of the build material has alreadystarted, starts or may be started by introducing energy upon theapplication of the build material, and a reaction-inhibiting orreaction-stopping liquid is selectively applied by the print head. 2.The method of claim 1, wherein the particulate material is selected fromthe group consisting of freshly mixed concrete or mortar, a mixture of afiller and a reactive resin system, a mixture of a foundry moldingmaterial and a reactive resin system common in foundries, a mixture of afiller and water glass or a mixture of particles and a meltablematerial, wherein the mixture is heated to a temperature above themelting temperature of the meltable material prior to application. 3.The method of claim 1, wherein an additional activation step takes placeafter the application from the print head.
 4. The method of claim 1,wherein the solidification reaction is delayed for hours.
 5. The methodof claim 1, wherein the solidification reaction is started by radiation.6. The method of claim 1, wherein the solidification reaction is ahydration, a polymerization or a phase transition reaction.
 7. Themethod of claim 1, wherein a cement, a gypsum, an acrylate a styrene, apolyurethane, an epoxy resin, a polyester or a polyamide is used as abinding agent.
 8. A material system for 3D printing wherein the materialsystem a particulate build material for applying to a build space in adefined layer thickness in the form of a powder or a dispersion, aliquid for applying via a print head to the particulate build materialin one or more areas in which the particulate build material is not tobe solidified by a solidification reaction, and wherein the particulatebuild material is selected so that a solidification reaction of theparticulate build material has already started, starts or may be startedby introducing energy upon the application of the particulate buildmaterial, and the liquid is selected for inhibiting or stopping thesolidification reaction. liquid is selectively applied by the printhead.
 9. A device for carrying out the method of claim
 1. 10. A device,suitable for 3D printing, comprising: a movable building platform, acoater for applying a build material, which is a self-hardening materialmixture, at least one print head unit, at least one mixing unit, and atleast one cleaning unit for cleaning the coater.
 11. The device of claim10, wherein the device includes: at least one cleaning unit for cleaningthe print head unit and/or a receiving unit for receiving the buildmaterial.
 12. The method of claim 1, wherein the step of moving thebuild space includes lowering, raining or horizontally moving the buildspace, wherein the build space is moved by the layer thickness.
 13. Themethod of claim 1, wherein no binder material is selectively applied,and/or no sintering step is carried out.
 14. The method of claim 3,wherein the additional activation step takes place with the aid ofirradiation.
 15. The method of claim 14, wherein the radiation isinfrared radiation.
 16. The method of claim 4, wherein thesolidification reaction is delayed for multiple days by the liquid. 17.The method of claim 4, wherein the solidification reaction completelystopped by the liquid.
 18. The material system of claim 8, wherein theparticulate material is selected from the group consisting of freshlymixed concrete or mortar, a mixture of a filler and a reactive resinsystem, a mixture of a foundry molding material and a reactive resinsystem common in foundries, a mixture of a filler and water glass or amixture of particles and a meltable material, wherein the mixture isheated to a temperature above the melting temperature of the meltablematerial prior to application.